C密码流结果与预期不符

我试图理解以下代码导致加密字节数组为16字节的原因,如果 plainText 长度为8字节。我期望结果的长度也为8字节?

private static byte[] encrypt(byte[] key, byte[] plainText)
{
    try
    {
        using (MemoryStream ms = new MemoryStream())
        {
            DES des = new DESCryptoServiceProvider() { Key = key, IV = key };

            using (CryptoStream cs = new CryptoStream(ms, des.CreateEncryptor(), CryptoStreamMode.Write))
            {
                using(BinaryWriter bw = new BinaryWriter(cs))
                {
                    bw.Write(plainText);
                }
            }

            return ms.ToArray();
        }
    }
    catch (Exception e)
    {
        Logger.LogWarning(e);
        throw e;
    }
}